There is currently being developed for the U.S. Army a surveillance system known as the Platoon Early Warning System (PEWS). This system comprises a number of seismic sensors comprising geophones and magnetic detectors which are deployed on the ground around an area to be protected. The sensors are all connected to centralized circuitry via a wire or radio link. The central unit includes seismic frequency amplifiers designed to efficiently amplify seismic signals picked up by the geophones and caused by walking or crawling personnel and to operate an alarm if such signals indicate the presence of intruders. This PEWS system is equipped with an automatic gain control (AGC) system which lowers the seismic amplifier gain in the presence of high amplitude signals. The purpose of this AGC system is to lower the false alarm incidence, however an undesired side effect of the AGC system is to lower the detection range in the presence of large seismic noise signals. Seismic noise may originate from air or vehicular traffic which is picked up by geophones or even wind which results in tree movement. Further, even in the absence of seismic noise, PEWS geophones which are placed in areas of high soil attenuation of the seismic waves of interest, or in areas where the soil-to-geophone coupling is poor, will have limited detection range and will be virtually useless for intended purpose.
The present invention comprises apparatus which can be used to detect the presence of seismic noise of the type described and can also be used to check soil conditions so that the PEWS sensors can be placed where they will be most effective. Also, the apparatus can be used in practicing a novel method for determining the proper spacing of PEWS sensors or geophones so that the coverage of adjacent geophones will be such that there will be no blind spots nor any substantial overlapping coverage of adjacent PEWS geophones. Prior to the present invention, soldiers had no effective means to aid them in the placement of PEWS sensors.